1. Field of the Invention
The present invention relates generally to a solid state color imaging apparatus for use, for instance, in a video camera, and particularly concerns an improvement in a solid state color imaging apparatus having a plurality of color filter elements combined with solid state imaging sensors and a processing circuit for processing output signals from the solid state imaging sensors.
2. Description of the Prior Art
In a video camera, important characteristics are resolution, sensitivity and dynamic range. Hitherto, many efforts and proposals have been made to improve these characteristics. This is true of course of a video camera utilizing a solid state color imaging sensor in which photodiodes or the like are disposed in two-dimensions. This invention particularly relates to a solid state color imaging apparatus intended to achiever higher dynamic range and higher sensitivity in comparison with conventional solid state color imaging apparatus. Characteristics of improved dynamic range and sensitivity in a solid state imaging apparatus are created using a color video camera having a monolithic solid state imaging sensor, as one example.
The dynamic range of the monolithic color camera is determined by the image sensing characteristic and method of color separation. The dynamic range or S/N characteristic of the solid state imaging sensor is as follows. When there is no incident light to the solid state imaging sensor (hereinafter referred to as dark noise) is determined mainly by the noise of the output amplifier of the imaging sensor and the dark current value, and on the other hand, the noise when there is certain incident light to the imaging sensor is determined by the nonuniformity in sensitivity of the imaging sensor and by the shot noise of the imaging sensor.
Since the solid state imaging sensor is a kind of IC, miniaturization of size is desirable for the sake of higher manufacturing yield. Besides, in order to pursue higher camera resolution, high integration is required. As a result of the decrease of size and the higher integration, the above mentioned dark noise and the S/N characteristic which is determined by saturation noise become worse as a general rule. This is found in the shot noise, for example. Saturation signal intensity of a generally utilized solid state imaging sensor is about 2.times.10.sup.5 -10.sup.6 e, and shot noise intensity at that time is about 300-10.sup.3 e, and the S/N ratio is about 53-60 dB. When the degree of integration increases, for instance, as area of CCD decreases, the intensity of saturation signal becomes low. Provided that the saturation signal intensity is 5.times.10.sup.4 e, the shot noise becomes 2.times.10.sup.2 e, and the S/N ration decreases to 47 dB. That is, the video camera utilizing the solid state imaging sensor should have an image output with a better S/N ratio of the image sensor.
On the other hand, the high integration necessarily decreases the areas of the imaging sensor elements, and this obviously decreases efficiency of the photoelectric transducing, hence sensitivity. Accordingly it is necessary to intend to improve efficiency of the photoelectric transducing by utilization of higher transmittance in the color separation. As to filters which meet the above-mentioned purpose, use of complementary type color filters is known. FIG. 1 shows one known example of color filter arrangement in a single chip color imaging camera using white (W), yellow (Ye), cyan (Cy) and green (G) as the complementary type color filters. This system reads out photoelectrically transduced signals w and g through the color filters W and G in a first horizontal scanning, and reads out photoelectrically transduced signals y and cy through the filters Ye and Cy in a second horizontal scanning. These read-out signals w, g, y, and cy are directly passed through low pass filters to produce a luminance signal Y. On the other hand, computation of (w-g)+(y-cy) and (w-g)-(y-cy) are carried out, and thereby principal color signals R and B are separated, and together with the above-mentioned luminance signal a known composite color signal is produced.
The above mentioned conventional color separation method is defective in that it has a narrow dynamic range. This is caused by differences between transmittances of different color filters. FIG. 2 shows image sensoring characteristic and results of operations of (w-g), (y-cy) and (w-g)+(y-cy). As is obvious from the curves of FIG. 2, (w-g)+(y-cy) is proportional to incident light intensity until the saturation point of w, but the curve sharply goes down after the saturation of w. That is, the above-mentioned systems of W, Ye, Cy, and G filters has only a limited usable range until the saturation point of w. At that time, other signals y, cy and g do not saturate. Therefore, the luminance signal Y at the saturation point becomes as follows: EQU Y=w+g=y+cy=R+2G+B (1),
wherein R, G and B are signals of principal color light.
Now provided that R=G=B=1, the luminance signal Y can be represented as EQU Y=4=(4/3)w (2).
That is, the luminance signal at the saturation point in the above-mentioned conventional system becomes (4/3).multidot.1/2=2/3 of a value Y=w+w=2w which is the luminance signal when no color filters are used. In other words, in the conventional W, Ye, Cy and G filter system, the dynamic range is limited to 2/3 of the case when no color filters are inserted.
This is contrary to the need for a color separation system which is usable in a narrower dynamic range for the above-mentioned solid state imaging sensor.
In summary, a color separation method usable for a narrower dynamic range image sensor utilizing color filters of high transmittances is need for a color camera using the solid state imaging sensor. However, the conventional system is not advantageous in dynamic range for the high transmittance filters.